1. Field of the Invention
This invention relates to a urea production process by synthesis where ammonia (NH.sub.3) and carbon dioxide (CO.sub.2) are reacted under high pressure and at a high temperature to form urea, ammonium carbamate, water and unreacted compounds, and in which reacted effluents discharged from the said synthesis reactor are treated to decompose the carbamate and recover the unreacted compounds in order to recycle them to the reactor; more specifically, the invention relates to a urea production process with low energy consumption, high reaction yields and low residual content of unreacted material in the urea produced.
2. Description of the Prior Art
It is known that high reaction yields are favoured by a high ammonia excess (compared with the stoichiometric ratio), which require, however, a high reactor operating pressure. A high synthesis pressure is unfavourable to the efficient separation of unreacted compounds from the urea solution obtained. In consequence before the so-called "stripping" technology, in which the bulk of unreacted components are separated in steps operating at reactor pressure by using a stripping agent (NH.sub.3 and/or CO.sub.2), became known pressure was drastically reduced downstream the reactor to achieve the efficient separation of the unreacted material.
In stripping processes the reactor operating pressure has been drastically reduced, to the detriment of yields, to a compromise pressure in order to achieve the isobaric separation of the unreacted material by using a stripping agent.
Several processes have been recently described, aiming to combine the advantages of high reaction yields typical of conventional processes with the advantages of stripping processes.
Among the most recent processes of this kind, the following might be mentioned:
(A) U.S. Pat. No. 4,208,347 (Montedison); (B) Japanese Patent Application PCT/JP 70/00192 (Mitsui T. and Toyo E.); (C) British Patent Application No. 2028311 (Ammonia Casale SA); (D) Italian Patent Application No. 24357A/80 (Snamprogetti).
It should be stated in advance that the forerunner of the above patent documents should be considered to be British Patent No. 1.1185.944 (Chemico) in which treatment of the bulk of the solution discharged from a high yield reactor is in two steps in series (only the first or both isobaric with the reactor); the bulk of the carbamate is separated in the first step also with the help of fresh stripping NH.sub.3 and the residual NH.sub.3 is separated in the second step with the introduction of fresh stripping CO.sub.2.
The above processes according to (A), (B) and (C) have, downstream a urea reactor, two separation steps in series in which the unreacted compounds are separated "selectively": more specifically in (A) and (D) the bulk of the carbmate is separated in the first step and residual ammonia is separated in the second step using CO.sub.2 as stripping agent; in (C) the bulk of the ammonia is separated in the first step and the carbamate is decomposed in the second step possibly with the help of CO.sub.2 as stripping agent either in the second or in both steps. This is achieved by operating under critical conditions in both separation steps. In the process according to (B) there are two separation steps downstream a high-yield reactor, in which the unreacted compounds are separated. As in processes (A) and (D), a falling film exchanger is used in the first step, using NH.sub.3 as stripping agent. Similarly to (A) and (D), therefore, the bulk of the carbamate is selectively separated in the first step, while in the second step the residual reactants are separated, using CO.sub.2 as stripping agent: a falling-film exchanger is used in both steps. In process (D), as a variation from process (A), the second treatment step is not isobaric with the rest of the loop (reactor in a single step isobaric with the first treatment step).
In process (A) all the vapours (NH.sub.3 +CO.sub.2) obtained by decomposing carbamate (prevalently in the first step) are separated and recycled directly to the reactor (and, where the latter is in two sections, to the upper section), while the vapours obtained in the second step (residual free ammonia and stripping CO.sub.2 fed to the falling film exchanger) are fully condensed and recycled to the reactor (upper section in the case of a two-section reactor).
In process (B) the vapours separated in the two steps downstream a conventional high-yield reactor are mixed and condensed before being recycled in solution form to the reactor by means of an ejector. In patent (A), although a reactor in two superimposed sections is described in one of the alternatives, the two streams of material separated in the two steps in series downstream the reactor are both recycled to the main reactor (upper section in the case of a two-section reactor) or simply to the single piece reactor. Even if, as is known, the two-step reactor is adopted to exploit the concept (known per se) of using several reaction zones with different NH.sub.3 /CO.sub.2 molar ratios in order to optimize transformation yields, it does not solve satisfactorily from an economic point of view the important problem of heat balance control (operating temperature) in the two reaction zones; this problem, up to the present, has been the main obstacle preventing the effective application of these systems.
The heat balance problem becomes even more critical in high-yield reactors where it is necessary to operate with high NH.sub.3 excesses, involving a greater lack of heat.
According to processes (A) and (D), moreover, since all gaseous compounds from the second treatment step are condensed before being recycled to the reactor, it is absolutely imperative, by reason of the reactor's heat balance, that practically all the carbamate should be separated in the first step, thus obtaining a sufficient amount of CO.sub.2 in the gas stream recycled directly as such to the reactor, which ensures that sufficient heat is produced as reaction heat from forming carbamate. To achieve such selective separation of the bulk of the carbamate in the first step it is nevertheless necessary to use complex and expensive falling film exchangers and large quantities of stripping agent (NH.sub.3) which, as described above, must be expensively evaporated.
In effect in process (A) there is also compensation for the insufficient heat balance in the two reaction steps by introduction in both steps of fresh feed ammonia, preheated and/or evaporated, thus using up energy and involving complex controls. It should also be pointed out that part of the fresh feed ammonia must also be sent to the first reactor effluent treatment stage as stripping agent to decompose the bulk of the carbamate. Both reaction zones therefore have insufficient heat, so that all the fresh feed ammonia must be uneconomically preheated and/or evaporated.
In process (B) the problem of the reactor's heat balance is ignored since all the carbamate (exothermic reaction, hence main source of heat) forms outside the reactor.
In process (C) this critical aspect, which nevertheless conditions and defines the recycling system for the unreacted compounds from the two treatment steps, is not described (because outside or not homogeneous with the essential aspect of the specific treatment according to the invention).
Both in process (A) and process (B) the high synthesis pressure required to produce high yields heavily conditions the separation efficiency of the unreacted compounds in the two treatment steps operating isobarically with the reactor.